Generally, the above type of derailleur, as disclosed in Japanese Utility Model Publication Gazette No. Sho 56-49,832, is provided with a linkage mechanism comprising a fixing member, a chain guide and a pair of linkage members for supporting the chain guide to the fixing member, so that when the chain is shifted from a smaller diameter front chain gear to a larger diameter one of the multistage front chain gears, the inner guide plate is brought into contact at its inside surface with one side surface of the chain in engagement with the smaller diameter front chain gear to thereby urge the chain toward the larger diameter front chain gear and disengage it therefrom and engage with the smaller diameter one.
The chain guide, however, is formed in a circular arc extending longitudinally of the bicycle along the outer periphery of the front chain gear, with the linkage members being connected to the upper portion of the chain guide through pivot pins. Accordingly, the chain in engagement with the smaller diameter front chain gear moves along the inside surface of the inner guide plate and below and apart from the connecting portions of the chain guide and the linkage members, and is urged axially outwardly of the front chain gear and apart from the connecting portions when the chain is shifted to the larger diameter front chain gear.
Thus, although the chain guide is connected at the upper portion of the inner guide plate with the linkage members, the chain comes into contact with the inner guide plate at the position below and apart from the connecting portions with the linkage member, whereby when the chain is shifted from the smaller diameter front chain gear to the larger diameter one, play at the connecting portion of the chain guide and each linkage member is enlarged and also the inner guide plate deflects reversely to the direction of urging the chain. Hence, it is required to move the chain guide a greater distance axially of the front chain gear to that extent. As a result, the speed change efficiency of the front derailleur is deteriorated when the chain is shifted from the smaller diameter front chain gear to the larger diameter one.